Cause for Optimism: Low Frequency Observations of Active Stars

Rachel Osten
LOFAR and the Transient Radio Sky, Amsterdam, December 2008

Transcript

1. cause for Optimism:Low Frequency observations of Active stars Rachel Osten Space telescope science institute Dec. 16, 2008 lofar transients workshop
2. outline • Starting point: the Sun • Science of low-frequency stellar emissions • Previous stellar low-frequency observations, such as they are • Experience from Swift in stellar transients • Surveys vs. pointed observations
3. solar phenomenology ✦dominant emission sources appear over active regions; associated with magnetic structures ✦size of radio photosphere at low frequencies is larger: plasma layer higher in atmosphere ✦below about 1 GHz, dominated by coherent phenomena νp=9000 √ne(cm) Hz νB=2.8x106B (G) Hz ✦transient emissions related to flares observed at other wavelengths courtesy of Stephen White, UMd
4. solar burst phenomenology type I: narrow band events + broad continuum type II: slow drift high → low ν, harmonic structure MHD shocks generating plasma waves type III: fast drift high → low ν, electron-beam generated plasma waves type IV: broad-band continuum type V: broad-band continuum
5. solar burst phenomenology type I: narrow band events + broad continuum type II: slow drift high → low ν, harmonic structure MHD shocks generating plasma waves type III: fast drift high → low ν, electron-beam generated plasma waves type IV: broad-band continuum type V: broad-band continuum
6. type II bursts Type II burst seen by Green Bank Solar Radio Burst Spectrometer; http://www.gsrbs.nrao.edu interpretation: CME- driven MHD shock waves propagating through solar corona & wind timescales: tens of minutes, generally unpolarized ν< few hundred MHz: high altitudes rooted in CME-associated shocks, so relationship to solar flares not one-to-one
7. type III bursts isolated type III burst seen by Green Bank Solar Radio Burst Spectrometer; http://www.gsrbs.nrao.edu ✦flare-associated beams of electrons (0.1-0.5c) traveling through atmosphere ✦fast electrons arrive before slower electrons, creating bump-on-tail instability ✦generation of Langmuir waves, conversion to EM radiation range of νs, heights GHz →kHz low corona →IP space moderate degrees of polarization groups or isolated forward or reverse drifts
8. type III bursts isolated type III burst seen by Green Bank Solar Radio Burst Spectrometer; http://www.gsrbs.nrao.edu ✦flare-associated beams of electrons (0.1-0.5c) traveling through atmosphere ✦fast electrons arrive before slower electrons, creating bump-on-tail instability ✦generation of Langmuir waves, conversion to EM radiation for ν∼ νp≈9000 √ne dν =∂ν ∂ne ∂h ∂s dt ∂ne ∂h ∂s ∂t dν/dt=νvbcos ϑ/2λn range of νs, heights GHz →kHz low corona →IP space moderate degrees of polarization groups or isolated forward or reverse drifts
9. type III bursts isolated type III burst seen by Green Bank Solar Radio Burst Spectrometer; http://www.gsrbs.nrao.edu ✦flare-associated beams of electrons (0.1-0.5c) traveling through atmosphere ✦fast electrons arrive before slower electrons, creating bump-on-tail instability ✦generation of Langmuir waves, conversion to EM radiation for ν∼ νp≈9000 √ne dν =∂ν ∂ne ∂h ∂s dt ∂ne ∂h ∂s ∂t dν/dt=νvbcos ϑ/2λn range of νs, heights GHz →kHz low corona →IP space moderate degrees of polarization groups or isolated forward or reverse drifts
10. STORMs isolated type IV storm continuum seen by Green Bank Solar Radio Burst Spectrometer; http://www.gsrbs.nrao.edu ✦storms generally long-lasting (hours- days), broadband continuum events without discrete fine structures ✦ related to sunspot/ flare activity ✦can be up to 100% circularly polarized
11. nonthermal = potential low-frequency sources all nonthermal emitters b.c. of magnetic fields are capable of producing coherent emissions
12. early low-frequency stellar detections • Lovell 1969: YZ CMi (M4.5, d=5.9pc) @ 240 MHz (80 Jy), 408 MHz (6 Jy) Jodrell Bank MKI • Lovell et al. 1974: UV Cet (M5.5, d=2.6pc) @408 MHz (200 Jy) Jodrell Bank MKI • Kahler et al. 1982: YZ CMi @ 408 MHz (60 mJy) Jodrell Bank MKI • Davis et al. 1978: YZ CMi @408 MHz (120 mJy) MKIA
13. recent stellar low- frequency observations serendipitous detection of II Peg (K2 IV +dM, d=42 pc); van den Oord &de Bruyn 1994 YZ CMi (M4.5); Kundu & Shevgaonkar 1988
14. fine structure in coherent stellar bursts Arecibo Effelsberg Jodrell Bank (Güdel et al. 1989) simultaneous detection: bursts extraterrestrial in origin, cover large bandwidth interferometer detections (Bastin & Bookbinder 1987) also confirms astrophysical nature, structure over smaller bandwidths AD Leo (M4.5, d=5pc) 100% circularly polarized
15. fine structure in coherent stellar bursts Arecibo Effelsberg Jodrell Bank (Güdel et al. 1989) simultaneous detection: bursts extraterrestrial in origin, cover large bandwidth interferometer detections (Bastin & Bookbinder 1987) also confirms astrophysical nature, structure over smaller bandwidths AD Leo (M4.5, d=5pc) 100% circularly polarized
16. Osten & Bastian 2006 Osten & Bastian 2008 use these quantities to infer physical conditions in source, emission mechanism dν/dt=-52 MHz/s key measurable quantities: durations bandwidths drift rates Time (seconds since scan start) Frequency (MHz) dν/dt=-3,-5 GHz/s Observations of AD Leo with upgraded Arecibo
17. ←Frequency Time→ drifting bursts seen in large bandwidth, high time resolution stellar radio flares Osten & Bastian 2006 Wide-band Arecibo Pulsar Processor dν/dt= -6.7 GHz/s •durations ~few 10s ms (6000 km) •fractional bandwidth 5-15 % •drift rates compatible with plasma emission, exciter speed 0.1-0.5 c, positive & negative drifts •durations consistent with collisional damping in coronal source •Tb>1014K
18. ←Frequency Time→ drifting bursts seen in large bandwidth, high time resolution stellar radio flares Osten & Bastian 2006 Wide-band Arecibo Pulsar Processor dν/dt= -6.7 GHz/s •durations ~few 10s ms (6000 km) •fractional bandwidth 5-15 % •drift rates compatible with plasma emission, exciter speed 0.1-0.5 c, positive & negative drifts •durations consistent with collisional damping in coronal source •Tb>1014K All consistent with plasma radiation interpretation-- stellar equivalent of solar type III bursts
19. ๏ durations (2 ms) imply source size 600 km, Tb>1018 K ๏ same sign, magnitude of drift (-2.2 GHz/s) ๏ fractional BW Δν/ ν<0.5% implies Δne/ne <1% for plasma interpretation ๏collisional damping implies cool coronal source (1.5 MK), n10 ∼2 →optical depth from collisional absorption τ≈300 (λn/1010cm) ๏need λn<<1010 cm (inhomogeneity?) implies exciter speed ∼4000 km/ s, comparable to thermal sound speed. Hard to see how to get instability going under such conditions still AD Leo! Osten & Bastian 2008 The quality of the observations is now sufficient to discriminate emission mechanisms
20. pessimism? • no detections of flare stars from Clark Lake (20-125 MHz), Ooty Synthesis Radio Telescope (327 MHz) despite hundreds of hours on source • GMRT P-band constraints (RAO in prep.): no significant emission when not flaring • constrast with optical flare rates: Kulkarni & Rau (2006) estimate ~3x108 flares per year • dm bursts can be up to 500 times quiescent flux densities, duty cycles < 1%, timescales <1 s • complex relationship between optical flares and particle acceleration, plasma heating?
21. pessimism? • FIRST survey (Helfand et al. 1999)– 5000 deg2, 0.7 mJy limit @ 20 cm – 26 stellar radio sources, 16 are new detections • GBI @ 13 cm for a handful of active stars followed for several years shows large transient outbursts HR 1099 (K1V+G5V, d=28 pc)
22. pessimism? • Pointed L band VLA+ATCA observations of a few active stars show common flux variations with large amounts of circularly polarized radiation (~1/3 of the time; Osten et al. 2004), with flux densities well above FIRST survey limit • fine structure in bursts also seen (Slee et al. 2007) HR 1099 Osten et al. 2004
23. Swift experience with stellar flares II Peg (K2IV +dm) autonomous trigger Dec. 2005 Eflare=1037 ergs, F(0.8-200 keV)=10-8 erg/cm2/s, Lx=1033 erg/s (Osten et al. 2007)
24. occurrence rates of stellar X-ray superflares • multi-year radio surveys showing large gyrosynchrotron events suggest X-ray superflares should occur at a rate of a few per year per star • Ariel-V 5.5 yr survey of X-ray transients, 6/27 transients were RS CVn systems: • Schwartz et al. (1981) 11/yr >6x10-10 erg/cm2/s on II Peg (1032 erg/s) based on 66 days of observation of the binary • Pye & McHardy (1983) all-sky rate of 23/yr above 4x10-10, 2.3/yr above 4x10-9 based on 5.5 years of data • handful of stellar X-ray superflares based on pointed observations (ASCA, BeppoSAX) suggest large occurrence rate to be observed by chance
25. EV Lac (M4, d=5 pc) Swift trigger April 25, 2008 (Osten et al. 2009) F(0.3-100 keV) = 4x10-8 ergs/cm2/s
26. EV Lac (M4, d=5 pc) Swift trigger April 25, 2008 (Osten et al. 2009) F(0.3-100 keV) = 4x10-8 ergs/cm2/s TITLE: GCN CIRCULAR NUMBER: 8371 SUBJECT: Swift-BAT triggered on the flare star Algol DATE: 08/10/13 21:42:36 GMT
27. EV Lac (M4, d=5 pc) Swift trigger April 25, 2008 (Osten et al. 2009) F(0.3-100 keV) = 4x10-8 ergs/cm2/s TITLE: GCN CIRCULAR NUMBER: 8378 SUBJECT: Swift detection of flare on CC Eri DATE: 08/10/16 11:40:50 GMT TITLE: GCN CIRCULAR NUMBER: 8371 SUBJECT: Swift-BAT triggered on the flare star Algol DATE: 08/10/13 21:42:36 GMT
28. Pulsating radio behavior on VLM stars/brown dwarfs-- potential LOFAR source Hallinan et al. 2007 TVLM513-46546 M9V recent measurements of large-scale magnetic fields of several kG (Reiners & Basri 2007) in some VLM stars, coupled with new insights into emission mechanism from studies of Earth’s aurora, suggest that an electron-cyclotron maser emission is operating 0.1 Msun, 0.1 Rsun star with Prot ∼ 2 hours, showing pulsating radio bursts actual radio maps folded over the 2 hour rotation period
29. Pulsating radio behavior on VLM stars/brown dwarfs-- potential LOFAR source Hallinan et al. 2007 TVLM513-46546 M9V recent measurements of large-scale magnetic fields of several kG (Reiners & Basri 2007) in some VLM stars, coupled with new insights into emission mechanism from studies of Earth’s aurora, suggest that an electron-cyclotron maser emission is operating 0.1 Msun, 0.1 Rsun star with Prot ∼ 2 hours, showing pulsating radio bursts actual radio maps folded over the 2 hour rotation period artist’s conception of TVLM513 as a stellar pulsar
30. what to expect from lofar • discrete structures in frequency and time: careful to separate intrinsic drift from propagation effects. can be used to diagnose plasma conditions in stellar atmospheres • storm continuum: diffuse emissions from trapped electrons. correlate with flares at other wavelengths • timescales, drift rates expected to get larger as go toward longer wavelengths (based on analogy with Sun) • pointed observations will reveal behavior of well-studied stars: true stellar transient population may be large (or manageable)